Make extract_volume work with closed and mixed intervals and allow nearest value selection

[sloosvel]

Description

This PR expands the capabilities of extract_volume so that it works with closed and mixed intervals. It also adds the possibility to perform the extraction taking into account the coordinate values.

There is also a small fix in the documentation regarding a link that was not pointing to the right function.

Closes #1926

Link to documentation: https://esmvaltool--1930.org.readthedocs.build/projects/ESMValCore/en/1930/recipe/preprocessor.html#extract-volume

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[codecov]

Codecov Report

Merging #1930 (96d9446) into main (40e00e8) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1930   +/-   ##
=======================================
  Coverage   92.64%   92.65%           
=======================================
  Files         236      236           
  Lines       12402    12417   +15     
=======================================
+ Hits        11490    11505   +15     
  Misses        912      912           

Impacted Files	Coverage Δ
esmvalcore/preprocessor/_volume.py	90.43% <100.00%> (+1.43%)	⬆️

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[valeriupredoi]

nice! I'd like to see the explanation of whan a left/right open interval means in terms of the operation itself ie closed: <=/>= and open:</> but that's just nitpicking at this late stage before the freeze 🍺

[sloosvel]

No worries I can add now a short explanation

[valeriupredoi]

if you could that'd be nice - pls merge main into here too, hoping no merge conflicts. Cheers Saskia 🍺